Check that arguments are only moving locals or box contents in the mir verifier

[bjorn3]

See https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/stack.20slot.20reuse.20Q/near/305406997 for more context.

[RalfJung]

You tagged this unsized-fn-params, but the check should apply to all move arguments, right? The relation to unsized-fn-params is that those must always be move?

[bjorn3]

Yeah, that is why I changed the issue title.

[bjorn3]

You were talking about the label? I tagged it because I assume that clarifying these semantics is a blocker for stabilizing unsized_fn_params if we ever want to stabilize it.

[RalfJung]

Looking at the existing verifier, the call destination probably needs the same treatment.